Implantable medical devices are available to provide therapies for restoring normal cardiac rhythms by delivering electrical shock therapy for cardioverting or defibrillating the heart, in addition to providing cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator (“ICD”) senses a patient's heart rhythm and may classify the rhythm according to a number of programmable rate zones in order to detect episodes of tachycardia or fibrillation. Single chamber devices are available for treating either atrial arrhythmias or ventricular arrhythmias, and dual chamber devices are available for treating both atrial and ventricular arrhythmias. Rate zone classifications may include slow tachycardia, fast tachycardia, and fibrillation.
Upon detecting an abnormal rhythm, the ICD may select and deliver a therapy based upon detected rate and/or other programmable criteria, for example. Cardiac pacing may be delivered in response to the absence of sensed intrinsic depolarizations within a specified time window, referred to as P-waves in the atrium and R-waves in the ventricle. In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and possibly escalating to more aggressive therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by delivering high-energy shock therapy. Termination of VF in this manner is normally referred to as “defibrillation.”
In many available ICDs, a physician or operator has the ability to program particular anti-arrhythmia therapies into the device ahead of time, and a menu of therapy options is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber or chambers in which the tachycardia is diagnosed. After the initial therapy is delivered, a subsequent redetection of tachycardia may lead to a more aggressive anti-tachycardia pacing therapy, for example. If repeated attempts at anti-tachycardia pacing therapies fail, a cardioversion or defibrillation shock may next be selected. For an overview of tachycardia detection and treatment therapies, reference is made to U.S. Pat. No. 5,545,186 issued to Olson et al.
The objectives of determining proper placement of a right ventricular coil electrode and a defibrillation threshold have been accomplished in the past by installing the right ventricular electrode in the patient's heart, inducing ventricular fibrillation, and trying to treat the defibrillation at various voltages and locations until the proper location for the right ventricular electrode and the defibrillation threshold are determined. The ventricular fibrillation may be induced by a pulsing of the right ventricular coil electrode synchronized with the T-wave, by passing an alternating current through the heart, or by other means known in the art. The patient is under general anesthesia or is sedated but conscious throughout this procedure, but it is still a potentially traumatic and painful procedure, since the typical defibrillation pulse is 750 volts and several attempts may be required to determine proper lead placement and defibrillation threshold.